Composite panels of cellulosic particulate material, including panels of ligno-cellulosic particulate materials, are known. These composite panels are made by adhering small pieces or particles of cellulosic material, such as sawdust, wood fibers, and wood flakes into a larger sheet form. Other cellulosic materials may also be assembled into sheets in the same manner. Without limiting the generality of the term "cellulosic material", straw is another specific example.
Commercial products formed using these known technologies include panels such as plywood, particle board, fiber board, flakeboard, and various end-jointed, edge-jointed, and face-jointed structural members. During the production of particle board, fiber board and flakeboard, small pieces of wood are treated with adhesive and assembled into a mat. The mat is then compressed into a high density panel, typically in sheet form, by use of heat and pressure. These panels vary in thickness, for example from about one-eighth inch to about six inches. Typically, phenolic or other resins are used in making these panels with phenol-formaldehyde resins being a specific example. Panels produced using this latter resin release minute quantities of formaldehyde during curing of the resin and while the panels are in use. It is desirable to reduce this release of formaldehyde for environmental reasons.
Depending upon the size of the wood or other cellulosic materials used in the composite panel, the panels may contain voids and surface defects or irregularities. In general, the size of the voids increase with the size of the particles used in forming the panel. For example, because flakeboard is manufactured from relatively large pieces of wood (e.g. one-half inch to six inch long flakes of up to about two inches in width or more), the surface is rough when compared to particle board or fiber board, which are made from much smaller wood pieces. In spite of its rough surface, flakeboard has bending strength and stiffness values which are several times greater than those of particle board and fiber board. For many applications, it would be desirable to have a panel with the strength of flakeboard, and yet which minimizes surface voids.
As a specific example of one such application, when resilient floor covering, such as vinyl sheets, or ceramic tile are used for flooring, these materials typically must be installed on a smooth, flat surface. A builder generally attains a flat, smooth supporting surface by installing a thin board on top of structural flooring. This thin board or panel is termed "underlayment" and generally has consisted of one-quarter to one-half inch thick plywood, fiber board, particle board, or sanded flakeboard. Of course, the thickness may be varied for particular applications.
Of these options, plywood provides a very smooth surface and is also resistent to swelling when wet. However, plywood is generally more expensive than other underlayment materials and is made from resources which are becoming less available as trees suitable for making plywood veneers become more costly and difficult to obtain. In contrast, fiber board, particle board, and flakeboard are less expensive than plywood due in part because they are made from a more readily available raw material source, such as smaller trees. However, fiber board, particle board, and flakeboard swell drastically when wet. This can occur, for example, when these materials are used as underlayment in bathroom, entry, kitchen sink, or laundry room areas which over time may become wet with water that then seeps into the underlayment. When these materials become wet, they may then swell and damage the overlaying floor material.
In addition, flakeboard has a unique disadvantage when used for underlayment. That is, the surface voids in flakeboard tend to telegraph through vinyl floor covering. Thus, surface smoothness, which is inherently present in plywood, particle board, and fiber board, but not in flakeboard, is an extremely important property of underlayment utilized beneath flooring materials such as vinyl flooring. A smooth, void-free surface is also desirable in an underlayment panel because it facilitates the cleanup of construction debris after the panel has been installed. Even tiny pieces of sawdust or other particles lodged under vinyl flooring can show up as a bump or projection in the surface of this flooring. This is especially true for increasingly popular "perimeter attached" vinyl floor covering.
Unless it is sanded, flakeboard is generally not suitable as an underlayment for floor covering. Its surface contains far too many voids, and the normal thickness variability between panels creates ridges where the panels meet on a floor. These ridges tend to show through the floor covering. Sanding the flakeboard panels can eliminate the ridges. However, sanding involves the cost and time of an additional manufacturing step. In addition, as a practical matter, even extensive sanding does not eliminate voids between large flakes in flakeboard.
Another problem arising from the use of flakeboard as an underlayment arises from the tendency for pieces of bark within the flakeboard to stain a vinyl flooring overlay. Bark contains large amounts of extractable tannins, which can bleed through vinyl overlays and result in an undesirable stain. This problem is especially acute for "perimeter attached" vinyl floor covering.
In addition to floor underlayment, there are other applications where flakeboard could be used more readily if it had a smooth, void-free surface and simultaneously maintained its structural properties. A specific example is in the manufacture of office furniture where flat work surfaces must span sixty inches or more and still resist deflection. Sanded oriented strand board, a type of flakeboard with layers of flakes oriented in desired directions, has been used as a substrate in furniture applications. However, extraordinary steps typically must be taken by a furniture manufacturer to make the surface useable. A smooth surfaced oriented strand board would greatly facilitate its use in this application.
The inventors have also attempted to solve these problems by applying a phenolic resin to flakeboard and curing the resin. However, the applied resin tended to seep into the porous flakeboard and when cured did not fill the voids in the board.
Therefore, a need exists for a cellulosic composite panel having a desired surface treatment, and in particular for a smooth surfaced flakeboard panel. A need also exists for an effective method of forming such panels.